This proposal covers ongoing structural studies of "molecular chaperones"-proteins that participate in and modulate (re)folding, unfolding, suppression of misfolding, or stabilization of alternate conformation of proteins and nucleic acids. We have crystallized the components of a bacterial analog of the ATP-dependent eukaryotic proteasome, the 850 kDa HslV/Hs1U complex of H. influenzae, in forms suitable for high resolution structure determination. Crystals of the full complex, of the ATP-dependent chaperone Hs1U alone, and of the protease Hs1V alone are documented in the proposal. We have also crystallized an ATP-dependent RNA chaperone, the eukaryotic initiation factor eIF4a from yeast. We have collected MAD data to 2.8 E from crystals of SeMet-labelled eIF4a protein; we are currently building a model of the protein.